OP27.05: Customized versus population-based standards at a routine third-trimester ultrasound for prediction of SGA at birth

Abstract

Objective: Birth weight is an important predictive parameter for neonatal morbidity and mortality. Accurate estimation of fetal weight is therefore a valuable tool for determining further obstetric management. Commonly used weight formulae have a lack of accuracy. The aim of this study was to develop and to evaluate new formulae specifically designed for different ranges of fetal abdominal circumference (AC). Methods: This study included 5314 pregnancies. Inclusion criteria were singleton pregnancy, ultrasound examination with complete biometric parameters within 7 days before delivery, and absence of structural or chromosomal malformations. With regard to sonographic AC measurements, four subgroups were built (AC: ≤ 290 mm; AC: 291-330 mm; AC: 331-359 mm; AC: ≥ 360 mm). For each population, best-fit formulae were derived by forward regression analysis with standard biometric measurements as independent variables and birth weight and log(birth weight) as dependent variables, respectively. Finally, accuracy of the new formulae was compared with commonly used weight equations. Results: In all subgroups, except for AC measurements between 331-359 mm, the new weight formulae demonstrated significantly improved accuracy compared to commonly used formulae. Conclusion: Especially in the lower und upper ranges of AC measurements, specifically designed equations help to improve fetal weight estimation. OP27.05Customized versus population-based standards at a routine third-trimester ultrasound for prediction of SGA at birth Obstetrics, Hospital Clinic, Barcelona, SpainObjective: To compare customized versus population-based standards at routine third trimester US for the prediction of SGA at birth. Methods: A total of 455 consecutive routine US examinations at third trimester (30.0-35.6 weeks) were included. Estimated fetal weight (EFW) was calculated by Hadlock formula including biparietal diameter, head circumference, abdominal circumference and femur length. Both population-based and customised standards were applied to the EFW. Customised centiles were calculated by adjusting for maternal height, booking weight, parity, ethnic origin, gestational age at delivery and fetal sex. SGA was defined as a birthweight below the 10 th centile according local standards. Results: SGA was confirmed at birth in 55 cases. A total of 27 and 38 fetuses had an EFW below the 10 th centile according population-based and customized standards, respectively. Both standards showed a 4% false positive rate. However, sensitivity was 2-fold higher (47.8 vs. 23.9%) and positive likelihood ratio 1.9-fold higher (12 versus 6.1) for customized than for population-based standards. Conclusions: Application of customized standards to routine thirdtrimester ultrasound fetal weight estimation improves the prediction of SGA at birth. If the first ultrasound estimation was over 4000g, performed by a sonographist mid-wife, a second operator (Ob-Gyn), blinded to the first sonography, repeated the biometry within 24 hours. Two Hadlock formulas were used (F1:...